Ground Penetrating Radar (GPR) uses high-frequency pulsed electromagnetic (EM) waves, typically in the range of 10 MHz to 10 GHz to determine subsurface information. GPR measurements are typically performed in geologic, engineering, hydrologic, and environmental applications. For instance, GPR data may be used to map depth to bedrock, depth to a water table, depth and thickness of soil strata on land and under fresh water bodies, and the location of subsurface cavities and fractures in bedrock. Other applications include determining the location of subsurface objects such as pipes, drums, tanks, and cables, mapping landfill and trench boundaries, mapping contaminants, conducting archaeological investigations, inspection of brick, masonry, and concrete structures, roads and railroad studies, and determining ice thickness.
GPR provides subsurface information by detecting changes in EM properties (i.e. at least one of dielectric permittivity, conductivity, and magnetic permeability) due to the soil and rock material, water content, and bulk density of the ground within which the EM waves are directed. The subsurface information may be acquired by using transmitting and receiving antennas placed as close as possible to the ground surface for which subsurface information is desired. The transmitting antenna radiates EM waves that propagate in the subsurface. As the wave spreads out and travels downward, if it hits a buried object or boundary with different electrical properties, then part of the wave energy is reflected or scattered back to the surface, due to contrasts in the EM properties of the subsurface material while part of its energy continues to travel downward. These regions of contrast in EM properties may be referred to as reflection interfaces. Most GPR reflections are due to changes in the relative permittivity of material. The greater the change in relative permittivity at the reflection interface, the great the portion of signal that is reflected. In addition to having a sufficient electromagnetic property contrast, the boundary between the two materials needs to provide a sharp transition within a distance that is on the order of the pulse length or wavelength of the transmitted waves.
The wave that is reflected back to the surface is captured by a receiving antenna, recorded, and analyzed in real-time or offline for later interpretation. The receiving GPR antenna records the reflected waves over a selectable time range. During GPR data analysis, the depths to the reflection interfaces are calculated from the arrival times of the reflected EM waves as well as the estimated EM propagation velocity in the subsurface materials. The velocity is proportional to the inverse square root of the permittivity of the material.
GPR measurements may be made at fixed locations or continuously by pulling a GPR measurement unit by hand or with a vehicle. Conventionally, uniform spatial sampling is desired with a spatial sampling interval that is dictated by the particular GPR application, frequency, and the EM properties of the underlying surface materials. Accordingly, each time an antenna has been moved a fixed distance across the surface of the ground or material that is being investigated, an EM wave is transmitted, and the resulting reflected EM wave is received and recorded. A single record of a transmitted pulse and the resulting reflected EM wave is called a trace, and the spatial difference between measurements is called the trace spacing. The trace spacing is chosen as a function of target size and the objectives of the particular GPR application.
When conducting GPR data measurements, it is also important to select a frequency band for the transmitted EM waves that is optimized for a particular application. For instance, selecting lower frequencies for the transmitted EM waves provides greater penetration with less resolution while selecting higher frequencies for the transmitted EM waves provides less penetration with higher resolution. More specifically, a resolution of a few centimeters can be obtained with high frequency antennas (i.e. 1 GHz) at shallow depths, while lower frequency antennas (i.e. 10 MHz) may have a resolution of approximately one meter at greater depths.
The accuracy of the GPR measurement is due to a variety of factors. Accuracy can be affected by the location of the antennas, the tow speed, the coupling of the antennas to the ground surface, and variations in soil conditions. GPR data is also sensitive to clutter caused by boulders, animal burrows, and tree roots as well as other natural sources. Other sources of clutter include reflections from nearby vehicles, buildings, fences, power lines, and other man-made sources. Electromagnetic transmissions from cellular telephones, two-way radios, television, and radio and microwave transmitters may cause noise on GPR records. In these latter cases, shielded antennas may be useful to limit these types of reflections.
The GPR data is displayed such that it closely approximates an image of the subsurface, with the anomalies that are associated with the objects of interest located in their proper spatial positions. Different types of displays include single traces (an A-scan), 2D displays (a B-scan) and 3D displays (a C-scan). A single trace can be used to detect objects (and determine their depth) below a spot on the surface. The A-scan consists of the transmitted energy pulse followed by pulses that are received from reflecting objects or layers.
By moving the antenna over the surface and recording traces at a fixed spacing, a section of traces is obtained to provide the B-scan. The horizontal axis of the B-scan is surface position, and the vertical axis is the round-trip travel time of the electromagnetic wave. B-scan displays have become the normal mode of two-dimensional data presentation for GPR data and may include assigning a color (or a variation of color intensity) to amplitude ranges on the trace.
The C-scan is a 3D representation created from a series of 2D B-scans. The C-scan is fundamentally a block view of GPR traces that are recorded at different positions on the surface. Accurate location of each trace is critical to producing accurate 3D displays. Normally, 3D block views are constructed which may then be viewed in a variety of ways, including as a solid block or as block slices.
Simplifying the image, by eliminating noise and clutter is one technique for optimizing the interpretation of the recorded data. As is well known by those skilled in the art, image simplification may be achieved by assigning appropriate amplitude-color ranges, using a limited number of colors, as well as decreasing the size of the data set that is displayed as the complexity of the target increases. Further image simplification in cases of very complex (or multiple) targets may also be achieved by displaying only the peak values (maximum and minimum values) for each trace.